1. Field of the Invention
The present invention relates to a proton (hydrogen ion) or hydroxide ionic conductive solid electrolyte which is applicable to a fuel cell and the like. The present invention also relates to a method of manufacturing the above-mentioned solid electrolyte and electrochemical systems such as the fuel cell using the above-mentioned solid electrolyte.
2. Description of Prior Art
Electrochemical systems using the proton conduction solid electrolyte have been already put to practical uses for a fuel cell, dehumidifier, or electrolysis apparatus for generating hydrogen. In a solid polymer fuel cell, electric energy are generated by electric current due to an electrochemical oxidization as shown in the reaction formula (1) of hydrogen (H2) supplied to a negative electrode and an electrochemical reduction as shown in the reaction formula (2) of oxygen (O2) supplied to a positive electrode.H2→2H++2e−  (1)1/2O2+2H++2e−→H2O  (2)
Also in a fuel cell using a fuel other than hydrogen (H2) supplied to the negative electrode such as methanol, the fuel is oxidized electrochemically, thereby emitting protons.
As for the electrolysis apparatus, a hydrogen generating apparatus has been put to practical use. The hydrogen generating apparatus generates hydrogen due to the reactions reverse to the reactions as shown in the reaction formulae (1) and (2). The electrolysis hydrogen generating apparatus has an advantage that any hydrogen gas cylinder is not required, because high purity hydrogen is generated merely by supplying water and electric power. Further, the electrolysis hydrogen generating apparatus has another advantage that the solid electrolyte allows pure water (without electrolyte) to be electrolyzed. Further, the electrolysis method is being applied to generating hydrogen peroxide in accordance with the reaction formula (3) for bleaching in the paper manufacturing industry (Electrochemistry, 69, No. 3, 154–159 (2001)).O2+H2O+2e−→HO2−+OH−  (3)
In the dehumidifier, the proton conduction solid electrolyte film is put between the positive electrode and negative electrode, similarly in the fuel cell and hydrogen generating apparatus. When a voltage is applied between the positive and negative electrodes, water is electrolyzed to oxygen and proton in accordance with the reaction formula (4) at the positive electrode. The proton which moved through the solid electrolyte to the negative electrode combines with oxygen in the air, thereby generating water, in accordance with the reaction formula (5). Therefore, the humidity at the positive electrode is removed, as a result of movement of the water from the positive electrode to the negative electrode.H2O→1/2O2+2H++2e−  (4)1/2O2+2H++2e−→H2O  (5)
The dehumidification is also possible by electrolyzing water in accordance with the principle similar to that of the electrolysis hydrogen generating apparatus. Further, an air conditioning apparatus in combination with a water evaporation cold air supplier was proposed (Proceeding of the Heisei 12 Institute of Electrical Engineers of Japan National Convention, p 3373 (2000)).
The above-mentioned systems which have been put to practical uses utilize perfluorosulfonic acid ion exchange film represented by the Nafion film. Further, various sensors and electrochromic devices are based on the principles similar to those of the above-mentioned systems, wherein protons moves through the electrolyte between the positive and negative electrodes or oxidization and reduction electrodes. Therefore, the proton conduction solid electrolyte can be utilized. Feasibility of those systems utilizing the proton conduction solid electrolyte are now being studied.
The hydrogen sensor utilizes the change of the electric potential on the electrode due to the hydrogen concentration change. The solid electrolyte of which chief ingredient is, for example, polyvinylalcohol (PVA) was proposed (Sensors and Actuators, 11, 377–386 (1987)). Further, humidity sensor may be fabricated, by utilizing the electric potential change on the electrode or change in the ionic conduction.
The electrochromic device utilizes the coloring of WO3 negative electrode in accordance with the reaction formula (6), when an electric field is applied to the negative electrode. The electrochromic device may be applied to a display device or light shielding glass. Inorganic compound solid electrolyte such as Sn(HPO4).H2O was proposed for the electrochromic device (Bull. Chem. Soc. Jpn., 60, 747–752 (1987)).WO3+xH++xe−→HxWO3 (colored)  (6)
Other electrochemical systems utilizing proton conduction solid electrolyte are, for example, a primary battery, a secondary battery, a light switch and an electrolyzed water manufacturing apparatus. The nickel hydrogen battery as the secondary battery comprises a hydrogen storage metal alloy for the negative electrode, nickel hydroxide for the positive electrode, and an alkaline solution electrolyte. During the charge and discharge at the negative electrode, the protons are electrochemically reduced and oxidized and hydrogen is stored in the hydrogen storage metal alloy, in accordance with the reaction formulae (7) and (8).(charge) H2O+e−→H (stored)+OH−  (7)(discharge) H (stored)+OH−→H2O+e−  (8)
On the other hand, during the charge and discharge at the positive electrode, the nickel hydroxide is electrochemically oxidized and reduced, in accordance with the reaction formulae (9) and (10).(charge) Ni(OH)+OH−→NiOOH+H2O+e−  (9)(discharge) NiOOH+H2O+e−→Ni(OH)+OH−  (10)
Since the charge and discharge reaction of the secondary battery is based on the movement of proton or hydroxide ion, the proton conduction solid electrolyte can be utilized in principle. However, the alkaline solution electrolyte has been conventionally used for the secondary battery.
As for the light switch, yttrium (Y) was proposed for the negative electrode (J. Electrochem. Soc., Vol. 143, No. 10, 3348–3353 (1996)). Light is switched on and off (transmitted or not transmitted) by an electric field, because the Y electrode transmits the light, when the Y electrode is hydrogenated under the application of the electric field in accordance with the reaction formula (11).Y+3/2H2O+3e−→YH3+3OH  (11)
Although the proton conduction solid electrolyte can be utilized in principle in this system, the alkaline solution electrolyte has been conventionally used.
The electrolyzed water is the water which was electrolyzed and has different effects depending upon whether the water was oxidized or reduced. The electrolyzed water is used for drinking water, food water, cleaning water, or agricultural water, depending upon its good effect to health, sterilization power, cleaning power, or action to promote growth of the crops. Although the electrolysis is accelerated by the electrolyte dissolved in the water, the electrolyte should often be removed from the electrolyzed water, when used. On the contrary, it is not required to remove the electrolyte, when the solid electrolyte is used.
However, the perfluorosulfonic acid electrolyte used conventionally for the fuel cell, the electrolysis hydrogen generating apparatus, or the dehumidifier as the electrochemical system using the proton conduction solid electrolyte has a disadvantage that the manufacturing processes of the perfluorosulfonic acid electrolyte are complicated and the products are expensive. The mass production may lower the product cost, but the cost merit is limited. Therefore, a low cost alternative is being expected.
On the other hand, it should be noted that the alternative solid electrolyte should have a sufficient water-absorbing power, because protons are transported at a high velocity under the action of the water contained in a solid in the proton conduction solid electrolyte operating at the room temperature. Further, it should also be noted that the alternative electrolyte should be water-resistive, because it is used in humid atmosphere. In the conventional perfluorosulfonic acid electrolyte, the water absorbed around the highly hydrophilic sulfonic acid group transports the ion, while the polyfluoroethylene skeletal part maintains the water-resistance, chemical stability and high temperature durability.
One of the low cost and highly hydrophilic hydrocarbon polymer is PVA which is applicable to the hydrogen sensor and the like, if it is made proton-conductive by mixing phosphoric acid. Proton can move at a high velocity, due to a high water absorption of PVA. However, PVA has a disadvantage that it is not stable in the humid atmosphere, due to its dissolution in water.
Other water-resistive and highly hydrophilic materials are inorganic hydrous compounds. For example, a hydrous glass such as P2O5—ZrO2—SiO2 manufactured by the sol-gel method is highly proton-conductive, due to absorbing a large quantity of water. Further, it does not dissolve in water and is durable at a high temperature which is characteristic of inorganic compound (J. Electrochem. Soc., Vol. 144, No. 6, 2175–2178 (1997)).
However, the inorganic hydrous compounds have a common disadvantage that they are brittle and is difficult to make thin films suitable for the solid electrolyte purpose. Further, they have a disadvantage that the raw material of the sol-gel method is an expensive metal alcoxide, an organic solvent such as alcohol is used in the manufacturing processes. Therefore, it is difficult to manufacture them cheaply even from a point of view of the manufacturing facilities. Although Sn(HPO4).H2O as employed in the electrochromic device is easy to be coated in the form of powder, it is difficult to make a film of Sn(HPO4).H2O which has a gas diffusion suppressing function and high strength which are required for the fuel cell and the like. Further, although molybdophosphoric acid (H3MoPO40.29H2O) and tungstophosphoric acid (H3WPO40.29H2O) are reportedly highly conductive (Chem. Lett., 17 (1979)), they are also poorly processed.
The hydrophilic organic polymer may be combined with the inorganic compound in order to overcome their respective disadvantages. For example, a proton-conductive material wherein silicon compound is chemically bonded in nanometer range with polyethylene oxide was disclosed in JP 2000-90946 A. Polyethylene oxide is as cheap and hydrophilic as PVA, but itself is dissolved in water. However, it reportedly becomes water-resistive and strong at a high temperature by mixing silicon compound by the sol-gel method. However, the composite material is difficult to be manufactured by any method other than the sol-gel method. Actually, any method other than the sol-gel method is not disclosed. Therefore, the composite material has a disadvantage that the cost of the raw material or the manufacturing cost is not reduced easily. Further, other composite material comprising a silicon compound, an inorganic proton-conductive agent such as tungstophosphoric acid, or molybdophosphoric acid and an organic compound such as polyethyleneoxide manufactured only by the sol-gel method is disclosed in JP 2001-35509 A.
Further, the conventional solid electrolytes are acidic and the electrode and other parts are limited to acid resistant material such as noble metals. Therefore, the electrochemical system as a whole does not become cheap. Further, acidic solid electrolytes make it difficult to develop some of the applications such as a primary battery, secondary battery and light switch, while the alkaline liquid electrolyte conventionally used in the primary and secondary batteries and the like is in danger of leaking outside.